<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="https://unpkg.com/dayjs@1.8.21/dayjs.min.js"></script>
</head>
<body>
<script>
    let starttime = new Date(dayjs().startOf('month')).getTime()
    let endtime = new Date(dayjs().endOf('month')).getTime()
    for(x = starttime; x<endtime;x+=(3 * 24 * 60 * 60 * 1000)){
        let str = dayjs(x).format('YYYY/MM/DD')
        let day = dayjs(x).date()
        console.log(str, day)
    }
</script>
</body>
</html>